With the development of modern technique for making paper and the faster speed to coat paper, the requirements for kaolin used in coating paper become stricter. For example, the whiteness should be above 87%; layer structures be maintained; pH value be about 6-7; the particle size distribution (PSD) be as follows: the content of particles of less than 5 μm is 100%, the content of particles of less than 2 μm is 98%, the content of particles of less than 1 μm is more than 90%, and the average particle size is less than 0.5 μm; and the viscosity content is more than 70%. The viscosity contents of the kaolin products produced by the KCS Company of U.S, the Alphacote Company of U.S and the Amazon 88 of Brazil are 70%, 74.5%, and 74.4% respectively. Among these, the kaolin product of KCS for coating paper has a particle size distribution as follows: the content of particles of −3.5 μm is 100%, the content of particles of −2 μm is 99.4%, the content of particles of −0.5 μm is 92.7%; the whiteness is 84.5% and pH value is 7.5. While, in China, the viscosity contents of the kaolin products applied for coating paper are mostly about from 66% to 70% due to the formation of mineral ores and producing technologies (the value of viscosity content are typically used in kaolin industry to measure a concentration, it is specified as a percentage value when the viscosity is 500 mPa·s, and the higher the value, the lower the viscosity). The particle size distribution is not proper and whiteness of kaolin products is emphasized excessively. Hence, China imports a great amount of high quality kaolin products for paper coating from U.S, U.K, Brazil and other countries in the globe every year. The kaolin resources are extremely abundant in China, but most of them can not be applied in the paper coating directly. The problems, within which the viscosity content is the most difficult one to be solved, become the bottle-neck of the production and application of Chinese kaolin and must be addressed urgently.
Kaolin with a minimum formula of Al4(Si4O10)(OH)8, is a 1:1 type dioctahedral aluminosilicate comprising a sheet of aluminum atoms coordinated octahedrally with apical oxygen atoms and hydroxyls and another sheet of silicon atoms coordinated tetrahedrally to oxygen, which are coordinated with the same oxygen. One side of the layer structure of kaolin is strong polar hydroxyls and the other side is oxygen atoms. As we know, there are no electrons circling round the hydrionic nucleus and oxygen is a strong electronegative atom, so it is easy to form the hydrogen bond between two sheets, so that kaolin is hardly dispersed.
Rheological behavior of kaolin suspensions has been one of the major factors determining the potential usage of kaolin in the paper industry. Previous studies have indicated three methods to reducing the viscosity of the kaolin slurry.
(1) Mechanical Extrusion
Some researches reduce the viscosity of the kaolin products through changing the particle size, shape and the distribution by milling the kaolin slurry with medium such as zirconia spheres, resin spheres or the like.
China Patent CN1315601A provides a method to reduce the viscosity of the kaolin slurry. The crude ores are purified, and then mixed with water and the clay has a solid content of about 60%-85%; the slurry is placed into an equipment with the functions of kneading and extrusion; the slurry is kneaded and extruded for 10-60 minutes; and then the product is obtained. The viscosity content is improved from 50%-65% of raw ores to more than 68%.
The products treated with that kind of method are affected by the quality of ores greatly and can not eliminate the interferences of the impurities. The limited enhancements confine the application of the methods.
(2) Use of Chemical Additives to Reduce Viscosity
At present, this kind of method is most common and prevailing. Some chemical agents such as sodium silicate, sodium polyacrylate and sodium hexametaphosphate combining with the method to adjust the pH value of slurry are usually used as dispersants to reduce the viscosity and some other new surface active agents are tried also. China patent CN1528979A provides a process to reduce the viscosity and improve the whiteness of the kaolin for paper coating, wherein the slurry is treated by high-gradient magnetic separation, and then chemical bleaching and surface modification. The chemical bleaching is performed under the following conditions: sulfuric acid 1.7-1.9 kg/t, sodium dithionite (also known as sodium hydrosulfite) 6.2-6.6 kg/t, bleaching time 10-20 minutes; sodium polyacrylate (the agent for surface modification) 1.4-1.6 kg/t. This process of that patent mainly emphasizes on how to apply the methods such as the high-gradient magnetic separation and chemical bleaching to improve the whiteness of the kaolin. The reduction of the viscosity of the kaolin products just lies on the chemical agent, sodium polyacrylate, so this method has great limitation. In view of the physical indexes of kaolin products, the viscosity content varies within the great range of 70%-72%. As we know, it is significant on the paper coating even just a variation of about 1%.
The defects of this method is that kaolin ores from different areas require different dispersants to achieve the optimum effect, but there are too many kinds of surface active agents, so it is needed to do plenty of experiments to verify the results. Furthermore, it is difficult to improve the viscosity of the products even at the optimal factors.
(3) Modification of Kaolin Powder
The surface electric property, surface absorption and soakage of kaolin can be changed by physical and chemical methods and the experiments are operated according to the layered structure of kaolin and the properties of structural functional groups of —Si(Al)—OH, —Si—O—Al— and —Si(Al)—O. These kinds of researches are few and generally, the viscosity content of kaolin is reduced by modifying surfaces of kaolin using surface active agents and thus reducing surface energy.
In general, perfect and universally applicable techniques about the viscosity of kaolin have not established until now. Most of researches on the viscosity of kaolin stay at the stage of the theoretical study in addition that the mechanical methods and methods in which dispersants are added are capable of reducing the viscosity of kaolin to some extent.
To control the particle size of kaolin mainly depends on the equipments including mechanical milling besides the hydro cyclones and horizontal spiral classifiers.